In the next few weeks, Sound Transit plans to start tunneling through Capitol Hill as the next phase of construction starts on a subway from downtown to the University of Washington.

A 21-foot diameter boring machine nicknamed "Brenda" will do the work. Brenda was named as a tribute to the project manager's wife.

"He named the last tunnel boring machine Mary-Margaret, which was the name of his mother," said Glen Frank, quality control manager for the contractor, JCM U-Link Joint Venture, formed by Jay Dee Contractors of Michigan; Frank Collucio Construction Company of Seattle; and Michaels Corporation of Wisconsin.

On Friday, Sound Transit took reporters for a tour of the future Capitol Hill light rail station at Broadway and East Denny Way, on the northwest corner of Cal Anderson Park. Right now, it's a cavernous concrete shell about 70 feet deep. Brenda waits at the bottom like a giant mechanized worm.

Brenda is more than 300 feet long and weighs more than 800,000 pounds.

"Basically the tunnel boring machine is designed as very simple submarine," Frank said.

The machine will burrow through 40 feet of dirt per day until it reaches the Downtown Transit Tunnel at Pine Street, where it will be disassembled, loaded onto trucks, and hauled back to the Capitol Hill site. Then, it will bore the second tunnel.

That's the last time the machine will see the light of day. Once the second tunnel is completed, crews plan to remove the cutter-head and the machine's guts and entomb the outer shell within the tunnel's concrete lining beneath Pine Street.

Tunneling will last for the next 12 to 18 months. It will be a 24-hour operation. Residents aren't likely to hear the tunneling itself, although there will be trucks hauling dirt away from the station site.

So far, 800,000 cubic yards of dirt have been excavated to build the station's box. The tunneling machine will dig from 15 feet below the surface to about 130 feet.

The boring machine is equipped with an array of sensors and instruments to monitor the amount of dirt removed. Crews will watch the data closely to avoid over-excavation. The machine has a 10-person crew, with the operator working from a control center about 60 feet from the cutter-head.

"There is potential for some settling. We anticipate some settling, but it is very small. If we do what we're planning to in terms of monitoring performance of the machine, we shouldn't have any problems," said Rick Capka, construction manager.

Sound Transit encountered a number of problems with tunneling through Beacon Hill for Central Link light rail, which opened in 2009. Soils settled after construction by the contractor, Obayashi Corp., and opened up voids under several residents' properties due to over-excavation. A 21-foot-deep void opened in one resident's yard and Sound Transit wound up purchasing the property for $400,000, with the total price of the mitigation work reaching close to $4 million.

With lessons learned from the other hill, this time will be different, Sound Transit says.

"It is different in that we have more oversight," Capka said. "We also have different technology."

The excavated dirt is carried out on a conveyor belt as the machine presses onward. Massive prefabricated concrete rings with rubberized gaskets are left in place to form the tunnel. Brenda is an "earth pressure balance" tunneling machine, meaning excavated dirt is used to fill a chamber behind the cutter-head to counterbalance the pressure it's up against as it grinds away.

Two other tunneling machines will start from the University of Washington to build the segment between campus and Capitol Hill. Together, they'll complete a 3.15-mile, $1.9 billion line. It's expected to serve an additional 70,000 riders.